1. Field of the Invention
The present invention relates to a capacitor having a changeable dielectric capacity and, more particularly, improvement in the reliability thereof.
2. Description of the Related Art
Nowadays, "network type" capacitors are known to a kind of capacitors. "Network type" capacitors are capacitors which have a changeable dielectric capacity.
An embodiment of a "network type" capacitor is shown in FIG. 1 in perspective view. FIG. 2 is a II--II sectional view of FIG. 1. FIG. 3 is III--III sectional view of FIG. 1.
Referring to FIGS. 1, 2 and 3, this capacitor comprises a main body part 61 of ceramic such as barium titanate (BaTiO.sub.3) and terminal electrodes which are formed using a paste of mainly silver (Ag). These terminal electrodes consist of a common terminal electrode 62a and a plurality of face terminal electrodes 62b facing the common terminal electrode 62a. The common terminal electrode 62a is formed on a face of main ceramic body part 61 as shown in FIG. 2 and the plurality of face terminal electrodes 62b are formed apart on a face of main ceramic body part 61. Lead wires 63 are each connected to these terminal electrodes 62a and 62b.
After this wiring, the ceramic main body part is covered with a layer 64 of epoxy resin or the like which is self-extinguishing.
Meanwhile, size of typical "network type" capacitor will be referred to below. Since the typical "network type" capacitor has nine capacitor elements, the typical "network type" capacitor is about 24 mm in width and 10 mm in length. Meanwhile, the ceramic main body part requires that its thickness is more than 0.4 mm because of strength of breakdown. Typically, an interval P between two face terminal electrodes is from 2.45 mm to 1.778 mm (see FIG. 2). When the thickness of face terminal electrode is S mm, difference of P-S is from 0.5 mm to 0.7 mm (see FIG. 2).
However, the above-described "network type" capacitor has the following problems.
FIG. 4 shows this "network type" capacitor in equivalent circuit view. As shown in FIG. 4, the "network type" capacitor has four capacitor elements C1,C2,C3 and C4. Their respective dielectric capacity C is given by the following formula: EQU C=Es.times.E.sub.0 .times.S/T
wherein "Es" represents the relative dielectric constant of ceramic material, "E.sub.0 " represents the dielectric capacity of a vacuum, "S" represents the an effective dimension of the face terminal electrode influencing its dielectric capacity, and "T" represents the thickness of the main ceramic body part.
Referring to the above formula, each dielectric capacity depends on its relative dielectric constant "Es", its effective dimension of its face terminal electrode and the thickness of the main ceramic body part. Therefore, when the relative dielectric constant, the effective dimension and the thickness are all constant in the "network type" capacitor it is difficult to make a variety of dielectric capacities. In order to increase dielectric capacity without enlarging the effective dimension, it is necessary to thin the ceramic main body part. However, there is a limit to the degree of thinness of the ceramic main body part because the capacitor breaks down when the ceramic body part is too thin.